Mind you, everything I've read is that it a giant killer and I wouldn't pretend to know as much as Voldemort but if some think it's a bit laid back or lacks a bit of energy, you could always try giving the output stage some gain. Unity gain can sometimes have more feedback than optimum sonically but I'm in no way saying that's happening here. Just something to try for those that may feel that way.

It is actually intentionally designed to have as much feedback as possible, to minimize distortion in the output stage. Further improvement is achieved by running two NJM4556 buffers in parallel, also doubling the maximum current output. Regarding too much feedback, I (and probably the designer of the O2 as well) recommend reading this article by Bruno Putzeys.

I get the circuit and qualified my response. There are definitely cases where more feedback is not preferred and I never said this was one of them. I've heard it compared. There's also a difference between 'too much feedback' which I never said this had and subtle character change. While I'm a big fan of the designer and prefer his methology to a lot of the snake oil out there, I also know that there are subtle differences to be had by massaging a circuit. In fact he did the same before finishing this amp. Distortion levels were already below what would be considered audible when he still heard changes in part selections. If they also measured better, was it the measured distortion that was audible at a 1/1000 % or some other non measured effect on the circuit? Being concise with design is a big deal but the best designers I've seen are aware that they don't know everything nor pretend to. I'm also not a zero feedback guy and use Naim at home. (Plenty of feedback)

Without even hearing it, I think this amp is a great achievement and Voldemort deserves all the Kudos he gets. In fact I played around with one of those 4556 ebay amps he blogged about, got the offset to 1mv with gain at 3db, balanced the lamp diode across both rails for a balanced drain etc and it clearly sounds better than an e11 for instance. Part of the finalization were a few bypass parts which I'm generally not a fan of (prefer the right better ones) but this was cheap and cheerful with bits about. These parts wont change the measurements in any important way but are audible. I am neither a measurement (very important) nor a subjective zealot. Seems most folks pick a camp and hunker down.

Guys just a question:I am looking to buy a O2 units for myself as i am (as always) tight on budget and it seems that this great amp could be a good start ,even if a basic one,to drive a pair of HD650 that i will try to buying soon.So i have been looking in my town for an adapter and the only one i found was 14Vac with 500 Mah.

Does someone with a similar power adapter have experienced some overheating of the unit,taken the amperage?or it is best to find one with 200/300 Mah?

Thanks for the input guys.

PS:To be honest i am in doubt if i have to get more patience in waiting the awaited ODA....

Guys just a question:I am looking to buy a O2 units for myself as i am (as always) tight on budget and it seems that this great amp could be a good start ,even if a basic one,to drive a pair of HD650 that i will try to buying soon.So i have been looking in my town for an adapter and the only one i found was 14Vac with 500 Mah.

Does someone with a similar power adapter have experienced some overheating of the unit,taken the amperage?or it is best to find one with 200/300 Mah?

Thanks for the input guys.

PS:To be honest i am in doubt if i have to get more patience in waiting the awaited ODA....

He unit does get warm but what exactly do you mean by overheat? Mine gets relatively warm after 2 hours of usage but I can still hold it in my hand with no problem.

How warm does it get, can you hold it in your hand without burning your hand? After how long of connectivity do you experience this?

No,i dont have yet the O2,i needed to find it's weird adapter before being able to put an order on it.i hope when the ODA is out to be able to choose which one i will get,if the unreleased version wont be much more expensive.

By the way if get a little warm after 2 hour ,should be fine,i am just asking because of the various amount of different amperages (from 200 to 800 , if i remember correctly) availability and i remember somewhere some fellow recommending the lowest amperage possible to avoid the amp to get warm.Nonetheless i think 500 Mah for the only adapter i found should work properly.

No,i dont have yet the O2,i needed to find it's weird adapter before being able to put an order on it.i hope when the ODA is out to be able to choose which one i will get,if the unreleased version wont be much more expensive.

By the way if get a little warm after 2 hour ,should be fine,i am just asking because of the various amount of different amperages (from 200 to 800 , if i remember correctly) availability and i remember somewhere some fellow recommending the lowest amperage possible to avoid the amp to get warm.Nonetheless i think 500 Mah for the only adapter i found should work properly.

Antonio

ODAC is out. You can order it either as standalone or built inside the O2. As far as the PSU goes if you check the designers blog he has provided a detailed information of the recommened PSU for his amp. I cannot share you the link due to head-fi ban on his blog but if you google it I am sure you will find it.

But again O2 is not the best sounding AMP out there, it is the best measuring amp in its price range and honestly a great buy if you can build it yourself. But when you buy it pre-built and than add the P&H there can be better 'sounding' amps out there than the O2.

But what made me keep the O2 is simply great engineering, and complete lack of HISS when turning the volume knob.

So i have been looking in my town for an adapter and the only one i found was 14Vac with 500 Mah.

Does someone with a similar power adapter have experienced some overheating of the unit,taken the amperage?or it is best to find one with 200/300 Mah?

....

The voltage of the power adapter is what is related to how hot the two voltage regulators in the O2 get, not the current. 14VAC is perfect, as is 500mA. The designer's listed power adapter recommend on his latest BOM (Dec 2, 2011) is 14 - 20VAC. Originally he had specified 12VAC which proved a little too thin in cases with the amp heavily loaded and the power mains voltage from the wall socket being on the low side. The upper 18VAC - 20VAC adapter voltage range is where the voltage regulators start getting a bit toasty.

As for current, anything from 200mA on up is fine for most headphones, or 400mA on up for low impedance + low sensitivity cans (the case that draws the most current). The O2 can push up to 140mA per channel, plus some chip and part quiescient current overhead to make it about 200mA per channel max, for a maximum current requirement of about 400mA total for both channels. There is an additonal techy issue of the adaptor's transformer core going into saturation on each power line cycle due to the current spikes caused by the double half wave rectifier setup the designer used in the O2 power supply. Probably doesn't make any audible difference, but to avoid that add about 20% to the adapter's current rating. That takes the it up from 400mA to around 500mA for O2 absolute max load current requirements, so what you've found there is just fine.

I use 16VAC + 400mA on two of my O2s and 16VAC + 1A (1000mA) on the other. Works great. I would have chosen 14VAC though if I could have found that combination in the US at Mouser at the time, to reduce the voltage regulator's heat dissipation a bit more.

ODAC is out. You can order it either as standalone or built inside the O2. As far as the PSU goes if you check the designers blog he has provided a detailed information of the recommened PSU for his amp. I cannot share you the link due to head-fi ban on his blog but if you google it I am sure you will find it.

But again O2 is not the best sounding AMP out there, it is the best measuring amp in its price range and honestly a great buy if you can build it yourself. But when you buy it pre-built and than add the P&H there can be better 'sounding' amps out there than the O2.

But what made me keep the O2 is simply great engineering, and complete lack of HISS when turning the volume knob.

Sorry Audio,i meant the ODA (Objective Desktop Amp),a "Desktop" O2 version with no batteries and some more input -and/or output - options ,based on what John Seaber from JDS Labs. told me.

Unfortunately "who should not be named" is not giving much more info about when it will be released,but there should not be any big great sound improvement from the O2.Nonetheless i will wait some review when it will be on the market to have a chance to choose from those two.I wish i could build it by myself but i am very bad at soldering i will just throw out that money...but i don't deny i would like to try to build a second unit to get more knowledge about what each components do and how a simple amplifier basically make.

By the way along Head-Fi, his site is providing a lot of information and knowledge which is the reason i respect it very much.Unfortunately i am having some more important priorities and my geographical and economic position don't allow me to afford an hi-end amp and dac.

I was considering some amp-dac's like the Mini ,the Quattro and few more,but for 150 dollars ,at the moment ,is tempting me a lot because for that price many tell is a great sounding amp.

Quote:

Originally Posted by agdr

The voltage of the power adapter is what is related to how hot the two voltage regulators in the O2 get, not the current. 14VAC is perfect, as is 500mA. The designer's listed power adapter recommend on his latest BOM (Dec 2, 2011) is 14 - 20VAC. Originally he had specified 12VAC which proved a little too thin in cases with the amp heavily loaded and the power mains voltage from the wall socket being on the low side. The upper 18VAC - 20VAC adapter voltage range is where the voltage regulators start getting a bit toasty.

As for current, anything from 200mA on up is fine for most headphones, or 400mA on up for low impedance + low sensitivity cans (the case that draws the most current). The O2 can push up to 140mA per channel, plus some chip and part quiescient current overhead to make it about 200mA per channel max, for a maximum current requirement of about 400mA total for both channels. There is an additonal techy issue of the adaptor's transformer core going into saturation on each power line cycle due to the current spikes caused by the double half wave rectifier setup the designer used in the O2 power supply. Probably doesn't make any audible difference, but to avoid that add about 20% to the adapter's current rating. That takes the it up from 400mA to around 500mA for O2 absolute max load current requirements, so what you've found there is just fine.

I use 16VAC + 400mA on two of my O2s and 16VAC + 1A (1000mA) on the other. Works great. I would have chosen 14VAC though if I could have found that combination in the US at Mouser at the time, to reduce the voltage regulator's heat dissipation a bit more.

Wow bro,you are really into it,congrats for your in sightings.Yesterday i did research all the principal "electric components shops" avenue in my town,and i did only found the adapter mentioned above..I don't know why ,but it is a strange power requirement indeed.Obviously i don't know much more cause i am more into Knives and Pans...I was almost convinced i would have to contact a personal made adapter,ahaha.Glad to know it is the proper PSU.Do you have three units...

What do you guys expect for the desktop version?If there will not be great difference in sound i maybe will try to buy the Trans-portable version due to his batteries capabilities feature.And hopefully if it wont cost too much..

The thing that makes this inexpensive amp great is that is doesnt have any sound of its own...its totally transparent....you can hear the source good or bad and hear what your cans are in a neutral environmnet.

All the measurments that impacts the sound are beyond human hearing....

Anybody ever figure out what would be the effect if the O2 was redesigned with three 9 Volt batteries?

You could automatically say the charge would last 50% longer.

But as the workload on each battery would be less, does that make the battery more efficient for a given charge?

First of all, I don't understand what you mean by "efficient" in terms of the battery. Could you maybe reword that?

Secondly, having three batteries (in configurations that make sense) would alter the configuration of the amp. As it is, one battery's - terminal is connected to ground, and the other battery's + terminal is connected to ground, so the first is generating the positive rail while the second is responsible for the negative rail. How would you set up three batteries relative to ground? If three 9V batteries give 27V, how do you make that +13.5V and -13.5V? You'd need a rail splitter or some other solution.

Also, note that if the supply voltages are higher, then the amp will draw more power. In some portable amps (e.g. FiiO E11), a low-power mode actually just reduces the rails—that's how low-power operation is achieved.